The present invention generally relates to methods for verifying whether a cavity is hermetically sealed, such as when semiconductor wafers are bonded together to hermetically enclose a micromachined sensing structure. More particularly, this invention relates to an electrical verification technique and device for detecting moisture within a cavity enclosing a micromachine sensing structure as an indication of whether the sensing structure is hermetically sealed within the cavity.
Within the semiconductor industry, there are numerous applications that require bonding a semiconductor wafer to a second wafer or glass, an example being sensors formed by a silicon wafer (referred to herein as a device wafer) with a micromachined sensing structure (micromachine), which is capped by a semiconductor or glass wafer (referred to herein as a capping wafer). Examples of semiconductor sensors include yaw (angular rate) sensors, accelerometers and pressure sensors, each of which typically entails a cavity that encloses the micromachine between the wafers. Absolute pressure sensors require that the cavity be evacuated and hermetically sealed, while the performance of yaw sensors with resonating micromachines generally benefit if the cavity is evacuated so that the micromachine operates in a vacuum.
By the very nature of their operation, micromachines must be free to move to some degree, necessitating that the seal between the wafers is sufficient to exclude foreign matter from the cavity. A hermetical seal ensures that moisture is also excluded, which would form ice crystals at low temperatures that could impede motion of the micromachine. Accordingly, the integrity of the bond between the wafers is essential to the life of a semiconductor sensor. Various bonding techniques have been used for the purpose of maximizing the strength and reliability of the bond. For example, the use of adhesives, dielectrics such as glass frit, and solders as intermediate bonding materials has been suggested in the prior art. Silicon direct and anodic bonding techniques that do not require an intermediate material have also been used. As would be expected, the conditions vary under which each of these bonding techniques will reliably yield a hermetic seal.
While each of these sealing methods has found wide use, sensors are inevitably produced whose cavities are not hermetically sealed after the bonding operation. To reduce returns and field failures, devices with inadequate seals need to be identified following the bonding operation. Labor intensive visual inspections can be useful to screen out unsealed devices, but are expensive and compromised by the likelihood of human error. Various automated inspection techniques are also available, though each have limitations and are often expensive to implement in a large-scale assembly process.
It is an object of this invention to provide a method for determining whether a hermetically sealed cavity has been achieved between a semiconductor wafer and a capping wafer.
It is another object of this invention that such a method employs a PN junction diode to sense the presence of moisture within the cavity as an indication of whether the seal is hermetic or not.
It is yet another object of this invention that such a method can be readily performed in a large-scale assembly process.
It is still another object of this invention that such a method is useful to inspect semiconductor sensors with micromachine sensing structures.
In accordance with a preferred embodiment of this invention, these and other objects and advantages are accomplished as follows.
According to the present invention, there is provided a method and device for verifying whether a cavity enclosing a micromachined sensing structure between a pair of wafers is hermetically sealed. The invention entails an electrical verification technique and semiconductor device that detects moisture within the cavity as an indication of whether the sensing structure is hermetically sealed within the cavity.
The method of this invention generally entails forming a bare, unpassivated PN junction diode in a semiconductor substrate, preferably a device wafer having a micromachine sensing structure. For example, the PN junction diode can be formed by implanting a P-type region in an N-type epitaxial layer of the device wafer. The device wafer is then bonded to a capping wafer of any suitable material to enclose the PN junction diode and micromachine within a cavity defined by and between the wafers. Bonding can be achieved by a variety of methods, including silicon direct bonding, silicon fusion bonding, anodic bonding and glass frit. The reverse diode characteristics of the PN junction diode are then determined by causing a reverse current to flow through the diode. For this purpose, either a known voltage is applied across the diode and the reverse leakage current measured, or a known reverse current is forced through the diode and the voltage measured. According to the invention, while passivated PN junction diodes are essentially unaffected by moisture, the unpassivated junction diode of this invention will exhibit unstable current/voltage readings if sufficient moisture is present within the cavity, which indicates whether or not the cavity is hermetically sealed.
Using the method of this invention, processing of a semiconductor sensor is only slightly altered to include the unpassivated PN diode junction within the cavity, and testing after wafer bonding is a matter of measuring the voltage or current through the diode. Accordingly, the present invention provides a low-cost inspection method that is not labor intensive and can be readily implemented in a large-scale assembly process, yet is highly reliable at detecting sensors having cavities that are not hermetically in order to reduce returns and field failures.
Other objects and advantages of this invention will be better appreciated from the following detailed description.